docs(wallet): Improve docs on ChangeSet persistence requirements

[ValuedMammal]

This work in progress PR is meant to expand on and clarify the documentation for bdk_wallet::ChangeSet.

fixes bitcoindevkit/bdk_wallet#9

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• I've signed all my commits
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• I ran cargo fmt and cargo clippy before committing

[ValuedMammal]

@tnull Is there any additional info or insight that you think should be included in the docs regarding the wallet ChangeSet, WalletPersister, etc?

[tnull]

@tnull Is there any additional info or insight that you think should be included in the docs regarding the wallet ChangeSet, WalletPersister, etc?

Ah, thank you for this PR! I think it would be great if the compatibility guarantees of ChangeSet could be spelled out as explicitly as possible (probably in its own sub-section of the ChangeSet docs?). Is the type ever going to change? If so, what types of changes to the data type do users implementing their own persistence need to prepare for? How do they need to detect and handle them? Can users persist the sub-changesets individually? Will forward compatibility (i.e., downgrades) also be supported? etc.

[ValuedMammal]

Reopened bitcoindevkit/bdk_wallet#198